Cryptosporidium is a newly recognized human pathogen which is associated with severe, persistent enteritis in immunocompromised patients, particularly those with the acquired immunodeficiency syndrome (AIDS) and significant though self-limited illness in the immunologically normal host. As more physicians have looked for the pathogen, the number of reported cases of cryptosporidiosis in both immunocompromised and normal individuals has continued to rise. There is currently no known effective therapy for the disease. The factors of human host defense which are essential for successful eradication of Cryptosporidium have not been delineated. Few clues have been provided by study of the clinical and histopathologic features of the disease in man, primarily immunocompromised (AIDS) patients, and animals. Currently available data suggests that: a) both T and B-cell mediated immunity are functional in host defense against this pathogen, and b) cellular and humoral components of immunity act in close concert with the target cell: intestinal epithelium, to effect successful defense against this parasite. Laboratory investigation of cryptosporidial mechanisms of immunopathogenesis has been severely impeded by: a) lack of in vitro methods for cultivating the organism, b) the small size and complex life cycle of the parasite, and c) absence of a symptomatic small animal model of the disease. Over the past two years we have succeeded in developing a model for the study of Cryptosporidium-cellular interactions in vitro using oocysts derived from stool specimens of infected patients. We have also successfully adapted the model for cultivating Cryptosporidium in vitro. The intent of this project is to further refine our model and use it to investigate the effects of various immune and non-immune factors on the interaction of Cryptosporidium with its host target cell - the enterocyte. We hope thus to define key cellular and humoral factors of host defense which function to eradicate this parasite in the intact host and which are absent in the immunocompromised individual. These studies should further our understanding of the immunopathogenesis of cryptosporidiosis and also form the basis for novel approaches to successful management of this disease in the future.